In the electronics industry it is often desirable to cover or coat an existing refractory metal surface with a brazable or solderable surface. Applications for such a procedure, include but are not limited to, I/O pads, wire bond pads, C4's (Controlled Collapse Chip Connection), seal bands, to name a few.
Many methods are available and practiced in the industry to cover or coat an existing refractory metal surface with a brazable or solderable surface. The most commonly used approach for treating refractory metal surfaces in the microelectronic packaging business is to employ electroplating or electroless-plating of pure or substantially pure Ni (nickel) film from an aqueous bath which is at or near room temperature.
Nickel is generally the metal of choice for plating refractory metals because it can be made to bond well with any of the refractory metals. In addition, Ni possesses good wetting characteristics for subsequent bonding processes, such as brazing or soldering, and it has excellent corrosion characteristics.
Recently, a few high temperature, dry, halide transfer processes have been disclosed and subsequently used by the industry for the purpose of plating nickel on molybdenum or tungsten.
One method disclosed in U.S. Pat. No. 4,590,095 (Park) uses a pack cementation approach. The essential elements for pack cementation are a powder metal source, an activator, and an object to be plated. Basically, the elements are placed in a chamber and the object is buried in a mixture of the powder metal source, activator, and usually an inert ceramic powder, such as, alumina, and then heated to a high temperature to establish vapor transport. The process allows for mass transfer of the gas species. For the Park process pure nickel powder was used as the metal source and the activator used was ammonium iodide powder.
A departure from this pack cementation approach for a halide transfer process was disclosed in U.S. Pat. No. 4,664,942 (Park). In this case ammonium iodide powder and pure nickel were still the essential elements for the halide transfer process. However, in this case nickel screens were used as the metal source rather than the nickel powder. And, the objects to be plated, containing exposed surfaces of refractory metal, were placed in stacks with the nickel screens acting as separators in the reaction vessel or work boat. The ammonium iodide activator for the process was held in a separate crucible within the work boat. The elements were again heated to a high temperature to establish vapor transport. The open nickel screen allowed for mass transfer of the gas species and also served as the nickel source.
Most recently, another improvement was put forward in U.S. patent application Ser. No. 08/668,295 (Reddy et al.), filed on Jun. 21, 1996, entitled "CVD OF METALS CAPABLE OF RECEIVING NICKEL OR ALLOYS THEREOF USING IODIDE", presently U.S. Pat. No. 5,869,134, issued Feb. 2, 1999 assigned to the assignee of the instant Patent Application, and the disclosure of which is incorporated herein by reference, where CuI powder was disclosed as a preferred iodide activator providing various advantages.
Another improvement that has been proposed in U.S. Patent Application Ser. No. 09/050,491, filed on Mar. 30, 1998, entitled "CVD OF METALS CAPABLE OF RECEIVING NICKEL OR ALLOYS THEREOF USING INERT CONTACT", assigned to the assignee of the instant Patent Application, and the disclosure of which is incorporated herein by reference, where at least one inert material is in a floating contact with the receiving metal, and the inert material provides physical separation between the source metal and the receiving metal.
DP (Dry Process) nickel process as it exists in prior art is basically a halide transfer process where nickel metal is transported in the gas phase from a solid nickel source and deposited as a solid metallic film on a refractory metal surface. The halide used in this case is a solid compound of iodide. Ideally the reaction will be made to take place in a closed container which in general is not tightly sealed.
In general, for halide transfer metal deposition processes, it is very desirable that the metal source material, preferably, a substantially pure solid nickel source, and the refractory metal area to be plated are kept in close physical proximity to each other. This close proximity condition is necessary in order to maintain a reasonable rate of metal deposition during the process.
In the prior art halide transfer processes, referenced above, the metal source material, powder or screens, were kept in close physical proximity to the refractory metal surface to be plated. However, in addition, due to the specific geometrical configuration of each assembly, the metal source material can at least at some point, also come into direct physical contact with the metal surface to be plated. It has been discovered that when the source metal and the target areas to be plated do touch each other, during the deposition process, while using these known processes, they, the source and the sink, can weld together and form a bond. When the nickel plated part and the other assembly materials are subsequently separated, after the deposition process has been completed, a defect in the deposited nickel film can be readily observed. This defect can take the form of a taffy pull of metal or a piece of metal debris or a missing section of the deposited nickel film, etc. This condition will normally, result in the rejection of the part or work piece.
Another improvement that has been proposed in U.S. patent application Ser. No. 09/050,490, filed on Mar. 30, 1998, entitled "CVD OF METALS CAPABLE OF RECEIVING NICKEL OR ALLOYS THEREOF USING AN INERT STRUCTURE WITH EMBEDDED NICKEL OR ALLOYS THEREOF", assigned to the assignee of the instant Patent Application, and the disclosure of which is incorporated herein by reference, where at least one source metal is secured to at least one inert material and where the source metal is in a floating contact with the receiving metal, and the inert material provides physical separation between the source metal and the receiving metal.
The present invention, however, teaches an improvement over the prior art, where an iodide source which is free of any extraneous metal contaminants is used in the DP nickel process. Here, the iodide source in a new physical form, namely gaseous form is used in the manufacturing process. This provides additional process controls such as pressure and concentration, and flow and gas phase distribution of the source metal, such as, nickel, through the inert structure for improving the reliability and yield of the halide transfer process where the source metal, such as, nickel metal, is electrolessly deposited onto a refractory metal surface.
With the method of this invention the metal source and the refractory metal surface to be plated, are kept in close physical proximity, as required, to effect rapid deposition rate which is also enhanced by the gas phase iodide source such as hydriodic acid and the reaction pressure above ambient. In addition, the inventive process allows substantial lowering of the transfer reaction temperature. The lower deposition temperature allows lower cost processing, which in turn improves the reliability of the coating. Furthermore, the lower process temperature also reduces the undesirable reactions of the materials in the pressure chamber.
It is preferred with the method of this invention that the high strength inert standoff material embodies or is bonded or attached to the metal source, so that the high strength inert standoff material and the metal source can be handled conveniently as a single unit. Preferably, the inert standoff material is in contact with, or in close proximity to, the receiving metal surface.
During the process the inert material continues to provide complete physical isolation between the source metal and the surface being plated, such that there is no opportunity for the source metal and the receiving metal to touch and weld or to form a bond creating a defect.
Furthermore, in this present invention the gaseous species is the accelerator and hence can accelerate and control the gas phase reaction. This will sweep the source metal surface in a continuous manner and will influence relatively faster kinetics at relatively lower temperature, thus the deposition kinetics can further be accelerated by increasing the pressure and/or temperature, etc.